The present invention relates to materials to be printed on, i.e. printing materials, useful for printing processes such as gravure printing or like intalgo printing, offset printing or like planographic printing, letterpress printing and hot stamping and also for electrostatic printing. The invention further relates to a method of fixing images to the material for use in preparing copies by electrostatic printing.
The term "printing" as used herein and in the appended claims not only refers to the conventional printing processes wherein an image pattern bearing plate is prepared from an original illustration or document and ink is transferred from the plate onto the material to be printed by application of pressure, but also embraces electrostatic printing or copying processes wherein a colored powder, i.e. toner, is deposited on the material to be printed, electrostatically without the application of pressure and is further fixed thereto with heat.
Plastics heretofore known for use as printing materials are polyvinyl chloride, ethylene-vinyl acetate copolymer or like vinyl resin, polyethylene, polypropylene or like polyolefin resin, polyester resin, styrol resin, acrylic resin, etc. These resins are used singly, or in the of form of composite materials in combination with a base material such as paper, wood or plastics of different kind to provide the surface of the composite material to be printed on. Printing materials prepared from these resins are printed to provide prints which are used as book covers, wrappers and wallpapers and for various other products.
The conventional plastics printing materials are produced generally by processing such a resin into a film, laminating the resin to a base material or coating the base material with the resin. The material prepared under the conditions best suited to the contemplated printing process is used. Of the plastics printing materials, the material made of polyolefin resin is generally low in printability. Especially for use in electrostatic printing, this material is poor in polarity, is low in compatibility with the vehicle of the toner deposited thereon and therefore encounters difficulty in giving copy images with good stability. Accordingly, the material has the drawback that the printing surface must be modified chemically or physically and thereby improved in printability. Since polyvinyl chloride usually has incorporated therein a plasticizer for giving flexibility, the printing material of this resin has the drawback that the inks usable for printing are limited or that the sheets of this material cannot be held placed one over another owing to the presence of the plasticizer which bleeds with time. The print prepared using this printing material fails to remain stable with time, permits bleeding of the plasticizer which is liable to obscure or dislodge the printed image, and becomes smeared by other print, such as newspaper, placed thereon. The print is further not preservable permanently owing to UV-degradation. The ethylene-vinyl acetate copolymer used is usually one having a low vinyl acetate content in view of the softening point of the resin and the strength of the film or sheet prepared therefrom. However, the printing material made of such resin of low vinyl acetate content is low in flexibility and elasticity, accordingly fails to come into intimate contact with the printing plate and encounters difficulty in giving satisfactory prints. When the vinyl acetate content is increased to afford higher flexibility, the resin exhibits a lower melting point and is not processable properly, giving a film or sheet of lower strength. The printing material obtained releases the disagreeable odor of acetic acid with lapse of time and is not usable satisfactorily. The printing material made of polyester resin requires a chemical or physical surface treatment so as to be given improved printability. Since the resin per se has a high softening point and is hard and low in elasticity and adhesion, it is difficult to laminate the resin to other base materials and it is difficult to blend the resin with other resins owing to poor compatibility. Styrol and acrylic resin are hard, brittle and poor in adhesion, are not compatible with other resins and therefore cannot be universally used as printing materials.
Remarkable advances have recently been made in copying techniques, and the conventional monochromatic (black-and-white) copying operation is being changed over to full-color copying operation. Monochromatic copies are prepared usually by transferring black (carbon black) toner images onto a copying material. Color copies are made using toners of three colors, i.e. red (magenta), yellow (azo type) and blue (cyanine type), and in addition, black (carbon black) toner, that is, four kinds of toners. Such toners are superposed on a copy material to complete a copy with the color of the original reproduced with high fidelity.
With color copying techniques, the color or tone of the original is separated into three colors utilizing electronic techniques, and the color patterns are read as by a computer and are reproduced with transferred toners as superposed to reproduce the color of the original. Theoretically, black can be produced using the three colors, while it is also practice to add the black toner finally. Accordingly, when the image of the original includes more intermediate colors or blackish colors, more toners of different colors are superposed. Especially, the black area is produced by superposed four toner layers. In the area where different toners are superposed in a multiplicity of layers, the toner image is not always fixed firmly when instantaneously heated in the copying machine. This problem is experienced with the use of sheets other than the paper specified for plain paper copying (hereinafter abbreviated as "PPC"), especially plastics composite printing sheets which are not amenable to the adhesion of toners. If the copy is folded, crumpled or strongly rubbed, the copy image dislodges to expose the white surface of the sheet to impair the copy, so that the copy is not fully useful. Such a problem is experienced also with monochromatic copies although to a different extent.